An intraluminal prosthesis is a medical device used in the treatment of diseased body vessels, including blood vessels. An intraluminal prosthesis is typically used to repair, replace, or otherwise correct a diseased or damaged blood vessel. An artery or vein may be diseased in a variety of different ways. The prosthesis may therefore be used to prevent or treat a wide variety of defects such as stenosis of the vessel, thrombosis, occlusion or an aneurysm.
One type of intraluminal prosthesis used in the repair of diseases in various body vessels is a stent. A stent is a generally longitudinal tubular device formed of biocompatible material which is useful to open and support various lumens in the body. For example, stents may be used in the vascular system, urogenital tract, bile duct, esophagus, trachea, colon, biliary tract, urinary tract, prostrate and the brain, as well as in a variety of other applications in the body. Endovascular stents have become widely used for the treatment of stenosis, strictures and aneurysms in various blood vessels. These devices are implanted within the vessel to open and/or reinforce collapsing or partially occluded sections of the vessel.
Stents generally include an open flexible configuration. This configuration allows the stent to be inserted through curved vessels. Furthermore, this configuration allows the stent to be configured in a radially compressed state for intraluminal catheter implantation. Once properly positioned adjacent the damaged vessel, the stent is radially expanded so as to support and reinforce the vessel. Radial expansion of the stent may be accomplished by inflation of a balloon attached to the catheter or the stent may be of the self-expanding variety which will radially expand once deployed. Structures which have been used as intraluminal vascular grafts have included coiled stainless steel springs; helically wound coil springs manufactured from a heat-sensitive material; and expanding stainless steel stents formed of stainless steel wire in a zig-zag pattern. Examples of various stent configurations are shown in U.S. Pat. Nos. 4,503,569 to Dotter; 4,733,665 to Palmaz; 4,856,561 to Hillstead; 4,580,568 to Gianturco;  4,732,152 to Wallsten; 5,395,390 to Simon et al., 5,234,457 to Andersen et al. and  4,886,062 to Wiktor.
Stents have been formed from a variety of techniques. For example, a stent made be made from a wire by winding or braiding the wire around a mandrel into a complex configuration, welding the wire at certain junctions, and heat treating the wire to create the implantable stent device. Alternatively, a stent may be made from a tube or sheet by stamping, cutting or etching a pattern into the starting material, expanding and/or rolling the starting material into a suitable stent shape, and heat treating to create the final device. Furthermore, stents can be produced by deposition, such as vapor deposition or electrochemical deposition, of metal onto a cylindrical mold. In addition to these methods stents have been made by knitting wires onto a cylindrical mandrel through use of a circular knitting machine.
Excluding the helically wound coil springs, these various stent configurations have an open lattice structure where the lattice segments are a single wire or single metallic structure. Different lattice segments from one structure may be welded to lattice segments from another structure to form the stent. For example, U.S. Pat. No. 5,395,390 to Simon et al. describes a stent formed from a single wire where the wire is arranged is a plurality of hexagonal cells. Abutting portions of the hexagonal cells are welded to one and the other for form the stent. Such placement of the wire to form the hexagonal cells and the selective welding are complicated and costly manufacturing processes.
One possible way to avoid the need for welding portions of a wire stent to form the stent's open lattice structure is to circularly knit a wire into an open lattice configuration. For example, U.S. Pat. No. 5,234,457 discloses a stent formed from a series of loosely-interlocked knitted wires. The knitted wires define the open lattice structure of the so-formed stent. The different segments forming the open lattice structure are straight laced wires without any interlooping of the wire along intermediate portions of the segments.
All of these manufacturing techniques, however, are quite complicated, making the resulting stent difficult and costly to produce. For example, to produce bifurcated stents, which have two branch stent portions extending from a main stent portion, individual stent portions are produced by the above-described methods and are subsequently and mechanically joined together. Alternatively, bifurcated stents have been produced by circular braiding or circular knitting techniques by forming the main stent portion on a mandrel, removing the stent from and mandrel, transferring the removed stent and the wire spools used to form the stent to a different circular braiding or circular knitting machine, spooling one set of wires into the machine to produce a branched stent portion on its corresponding mandrel and spooling another set of wires into the machine to produce a branched stent portion on its corresponding mandrel. While this latter technique avoids the problem of mechanically joining different stent portions, the required use of different mandrels about which different stent portions must be produced complicate the manufacturing of such stents. Similar complex manufacturing problems even exist even for producing a stent that has just a variable shape, such as a contoured stent or a stent with a varying diameter.
Moreover, stents having their individual lattice cells being formed from a single wire or member have somewhat limited flexibility to vary their shape, or can vary one dimension, such as diameter, only at the expense of another dimension, such as length. For example, stents are often radially expanded during implantation into a bodily lumen. The stents have an open lattice configuration so that the figuration can be somewhat altered to permit, among other things, the radial expansion of the stent. Such changes in the open lattice configuration typically result in a foreshortening of the stent upon expansion because the individual segments forming the lattice are rather unyielding.
Thus, there is a need for a stent having an open lattice structure with increased flexibility without the disadvantages of the prior art. Furthermore, there is a need for a method for producing varying shaped stents, including bifurcated stents, as unitary structures without the complex and complicated techniques associated with the prior art.